In recent years, optical fiber communication technologies are being developed rapidly, including a passive optical network (PON) to provide high speed broadband services to end users. A wavelength division multiplexing (WDM) PON, among others, provides a dedicated bandwidth of a logical point-to-point network between optical line terminal (OLT) at the telecommunication central office and a number of optical network units (ONUs) at the end users' locations. Generally, the ONUs and OLT search and choose their working wavelength to establish a network connection. In order to meet increasing demand, therefore, equipment manufacturers and network operators are continually looking for ways to improve efficiency in establishing the network connection.